The present invention relates generally to papermaking fibers and more specifically to a method of bleaching and providing durable curl to fiber by way of high temperature and pressure, low mechanical energy processing.
Refining and bleaching cellulosic fibers for papermaking is well-known. Various systems and processes are used for preparing pulps, including chemical pulping processes such as the Kraft process, mechanical processes, chemi-mechanical processes, thermo-mechanical processes and so forth. The art is appreciated by reference to the following patents and patent applications.
U.S. Pat. No. 2,008,892 to Asplund discloses an apparatus for refining wood chips into mechanical pulp provided with a grinding portion including a stationary disk, and a rotating disk.
There is disclosed in U.S. Pat. No. 2,516,384 to Hill et al. a process for mechanically curling cellulose fibers. The method of the ""384 patent includes forming the pulp in the presence of a limited amount of aqueous liquid into small, discreet nodules of fibers and causing the nodulated pulp to form into rotatable units and travel roll wise under compression, thereby subjecting the nodules to mechanical pressure with continuous reorientation of the nodules relative to the direction of applied pressure and thus imparting kinks, bends, and twists to the pulp fibers or fiber bundles. See Col. 4, lines 73 and following, through Col. 5, lines 1-20.
U.S. Pat. No. 3,023,140 to Textor discloses adding hydrogen peroxide and wood chips to a refiner for the purpose of simultaneously bleaching and refining the chips. (See FIGS. 2 and 3).
U.S. Pat. No. 3,382,140 to Henderson et al. is directed to a process for fibrillating cellulosic fibers. Cellulosic high consistency papermaking pulp in the form of a semi-solid, nonflowable and nonpumpable lumping mass composed of defibered fibers is continuously refined by passage through a refining space comprising opposed disk like working surfaces relatively rotatable about a common axis wherein the pulp is continuously maintained packed under high compression to cause defibrillation by interfiber friction along the surfaces of the individual separated fibers without substantially fracturing the fibers. In general, fibrous material is defibered and then dewatered to increase its consistency to a level where it forms a semisolid, nonflowable, moist mass adapted for high consistency refining. Pulp consistency in the range of between about 10% and about 60% with the fibers in intimate contact; preferably between about 20 and 35% is satisfactory. If the consistency is much below 10% (according to the patent) the amount of water present may act as a lubricant preventing the desired refining by inter-fiber friction. If much greater than 60%, the pulp will be too dry which may result in burning under the inter fiber friction. Examples of the ""140 patent teach mechanical power input of from about 5 to about 40 HP day/ton of pulp produced.
There is disclosed in U.S. Pat. No. 3,773,610 to Shouvlin et al. a pressurized system for pulp refining including pressurized double disk treatment. According to the ""610 patent, all fibrous materials are passed through a series of treatments under a steam pressurized atmosphere of from 10 to 150 psig and a temperature of between 115xc2x0 C. and 200xc2x0 C. in the absence of accompanying liquid. The raw fibrous materials are initially passed through a tube in which they conditioned by either the steam atmosphere, or by liquid chemicals under steam pressure, and then are passed between simultaneously rotating disks of a double disk refiner which is also under steam pressure. Subsequent to treatment with the disks the fibrous materials are passed to another conditioning tube, such as a digester or a bleach tower where they are further conditioned by liquid chemicals under the same steam pressurized conditions. The fibrous materials may thereafter be washed, cooled and/or pressed.
U.S. Pat. No. 3,808,090 to Logan et al. relates to a method of making wood pulp involving the mechanical abrasion of wood particles in the presence of water in an inert gaseous atmosphere. According to the process, wood particles are fed into a substantially closed chamber where they are mechanically abraided in the presence of water in an inert gaseous atmosphere (steam) at an environmental pressure of 10-60 psig, a temperature of 160xc2x0-300xc2x0 F. and under a power consumption of 50-150 HP day/ton. In the ""090 patent the Aspland process is characterized as suitable only for low quality pulp. It is noted that the conditions of the Aspland process are selected to provide mechanical reduction of the wood into fibers with the least possible energy input. To this end, high pressures of the order of 115-150 psig and relatively low energy input of the order of 7-12 HP day/ton are employed to obtain the best results. See Col. 1, lines 51-65.
U.S. Pat. No. 3,873,412 to Charters et al. relates to a method of mechanically refining a mixture of Kraft and semichemical pulp. The method is used for producing pulp for use in the manufacture of Kraft type products such as liner board and bag grade paper comprising the steps of steaming small segments of fibrous material, defiberizing the same in a pressurized atmosphere at an elevated temperature and, while the resultant fiber products are still hot, mixing them with hot Kraft pulp and then refining the mixture so obtained.
U.S. Pat. No. 3,948,449 to Logan et al. is directed to an apparatus for the treatment of lignocellulosic material. The ""449 patent also relates to the production of a mechanical pulp of improved strength properties. The lignocellulosic material is fed into a substantially closed chamber where it is mechanically abraided under a power input of 15 or more HP day/ton. During the abraiding step the material is maintained in an inert gaseous atmosphere at a pressure of 10-80 psig, preferably 20-40 psig. It is noted in the ""449 patent that the Asplund process is well known in the industry for the manufacture of low grade pulps for employment in the manufacture of roofing and flooring felts. The system involves generally presteaming wood chips followed by refining under high pressure. The products are not suitable for high quality or high strength papermaking because of their inherent low strength and other poor papermaking qualities.
U.S. Pat. No. 4,036,679 to Back et al. is directed to a process for producing convoluted and fiberized cellulose fibers and sheet products. The process includes the application of contortive forces to a pulp mass under controlled operating conditions, wherein the feed rate, work space gap and relative rate of movement of the working elements applying the contortive forces are correlated to maintain the work space filled with fibers under sufficient compression.
U.S. Pat. No. 4,187,141 to Ahrel et al. relates to the production of bleached wood pulp from wood chips using a disk refiner. In this patent it is disclosed to impregnate wood chips with an alkaline bleaching liquid prior to defibrating the chips in the refiner.
U.S. Pat. No. 4,409,065 to Kasser discloses a method of making an improved bag from Kraft pulp including a curlation step before web formation. The curlation step is preferably carried out promptly before the web is formed.
U.S. Pat. No. 4,431,479 to Barbe et al. is directed to a method for treating pulp fibers that have already been curled. The method includes subjecting the pulp to a heat treatment while the pulp is at a high consistency, thereby rendering the curl permanent to subsequent mechanical action. The permanent curl has advantages for paper machine runnability and for increasing the toughness of the finished product. During the process of papermaking most of the curl in both high consistency refined mechanical and high yield sulfite pulp is lost in the subsequent steps of handling at low consistency and high temperatures. See Col. 3, lines 20-29. In the ""479 patent the method of curling takes place at medium to high consistency (15%-35%) and may be a high consistency disk refining action as is generally used in pulp manufacture. Col. 4, lines 32-35. According to the ""479 patent, it is seen that the process is highly effective for ligno cellulosic pulp fibers, for example, mechanic pulp and high yield sulfite pulp fibers. The treatment reportedly has no effect on cellulosic pulp fibers which contain little or no lignin. Col. 8, lines 4-10. The heat treatment process described in the ""479 patent takes place in a digester at a temperature of about 150xc2x0 C. after the fibers have been curled. Generally, the method is reported useful for treating high yield or mechanical pulps which have been curled by a high consistency action which method includes subjecting the pulp to a heat treatment at temperature of 100xc2x0 C.-170xc2x0 C. for a time varying between 60 minutes and two minutes while the pulp is at a high consistency, 15%-35% to render the curl permanent.
U.S. Pat. No. 4,455,195 to Kinsley is directed to a fibrous filter media and process for producing it. The process involves selection of a lignin containing fiber source having a lignin content of at least about 10% and thermal mechanically pulping the fiber source under temperature/pressure conditions of 300xc2x0 F.-350xc2x0 F./50 psig-120 psig and a refiner energy utilization of about 8-35 HPD/ADT. The thermal mechanically produced fibers are characterized by a high degree of stiffness and an extremely smooth surface free of fine fibril formation and thus are substantially non-self-bonding.
U.S. Pat. No. 4,488,932 to Eber et al. discloses a method of making fibrous webs of enhanced bulk. See also European Patent Publication No. 0 101 319. Webs are produced by subjecting hydrophilic papermaking fibers to mechanical deformation, e.g. hammermilling sufficient to deform the fibers without substantial fiber breakage, dispersing the resulting curled or kinked treated fibers, preferably in admixture with conventional papermaking fibers in an aqueous medium, to form a fiber furnish, and forming a wet laid web from the resulting fiber furnish within a period of time, e.g. within five minutes, such that the deformations of the treated fibers are at least partially retained and impart enhanced bulk and softness to the finished fibrous web.
U.S. Pat. No. 4,548,674 to Hageman et al. is directed to a method of regenerating waste paper. Waste paper containing polymeric contaminants is broken down in the presence of an acidic aqueous solution containing at least one peracid. Particular peracids disclosed include permonosulphuric acid and peracetic acid.
U.S. Pat. No. 4,734,160 to Moldenius et al. discloses a method of peroxide bleaching lignocellulose-containing material for providing a pulp of both high strength and brightness. Increase in strength is provided in the first stage by hyper-alkaline peroxide bleaching pH of over 12. The desired brightness increase is provided in a subsequent stage with or without intermediate washing of the pulp at a lower initial pH.
U.S. Pat. No. 4,756,798 to Lachenal et al. teaches the concept of adding oxygen during the hydrogen peroxide bleaching of mechanical pulp. The bleaching liquid that is disclosed in this patent includes alkaline hydrogen peroxide with sodium silicate and magnesium sulphate.
U.S. Pat. No. 4,898,642 to Moore et al. is directed to twisted, chemically stiffened cellulosic fibers and absorbent structures made therefrom. According to the ""642 patent curled cellulosic fibers are chemically stiffened with a cross linking agent which is typically a C2-C8 dialdehyde.
U.S. Pat. No. 4,915,785 to Siminoski et al. discloses a single stage process for bleaching pulp with an aqueous hydrogen peroxide bleaching composition containing magnesium sulphate and sodium silicate.
There is disclosed in U.S. Pat. No. 4,938,842 to Whiting a bleaching liquid composition including hydrogen or sodium peroxide, sodium hydroxide, sodium silicate, magnesium sulphate and a chelating agent.
U.S. Pat. No. 4,976,819 to Minton discloses a method for treating pulp prior to forming a web. The method includes mechanical treatment of a pulp slurry of up to 50% consistency by dewatering and compacting the pulp. The pulp is twisted and kinked such that a web of enhanced softness is provided. The preferred device for imparting such twisting and kinking, is a plug screw feeder. Pulp that has been so treated exhibits increased drainability in a wet section of a paper machine.
U.S. Pat. No. 5,211,809 to Naddeo et al. discloses a color removal process for secondary (recycle) fiber. Color from dyes is removed from secondary pulps with non-chlorine based bleaching agents in treating sequences using oxygen with combinations of peroxide, ozone and/or hydrosulfite, at controlled pH conditions (less than 8 or greater than 10). Acid treatment prior to bleaching improves color removal and protects fibers from damage at more severe bleaching conditions.
There is disclosed in U.S. Pat. No. 5,244,541 to Minton a pulp treatment method wherein mechanically refined pulp is kinked and twisted and subsequently subjected to papermaking process steps.
U.S. Pat. No. 5,296,100 to Devic relates to hydrogen peroxide/alkaline bleaching of wood pulps. High-yield ligno-cellulosic wood pulps are bleached by pre-treating the pulp with a complexing agent and washing the pre-treated pulp followed by bleaching the pulp with hydrogen peroxide in an alkaline medium. When from about 60 per cent to 85 per cent of the initial amount of hydrogen peroxide has been consumed, a supplementary amount of hydrogen peroxide being equal to or less than the initial amount is added.
European Publication No. 0 440 472 reports high bulking resilient fibers produced by crosslinking wood pulp fibers with polycarboxylic acids such as citric acid.
U.S. Pat. Nos. 5,384,011 and 5,384,012 to Hazard et al. disclose a process for preparing individual crosslinked cellulosic fibers wherein curing and drying are carried out in separate stages. The drying and curing steps are carried out in turbulent pressurized superheated steam.
U.S. Pat. No. 5,501,768 to Hermans et al. is directed to a method of treating papermaking fibers for making tissue. According to the ""768 patent, the throughdryability of dewatered, but wet, sheets made from papermaking fibers can be significantly increased by subjecting an aqueous suspension of the fibers at high consistency to elevated temperatures with sufficient working of the fibers. It is noted in Col. 3, lines 36 and following that the temperatures can be about 150xc2x0 F. or greater. It is further noted that mechanical treatment with equipment having relatively high volume to working surface areas, such as dispargers are preferred and that disk refiners, for example, are not preferred. See Col. 3, line 65 to Col. 4, line 13. Power inputs are greater than 1 HP day/ton. Note examples 1-11. See, also, U.S. Pat. No. 5,348,620.
U.S. Pat. No. 5,571,377 to Tibbling et al. describes a process for peroxide bleaching of chemical pulp in a pressurized bleach vessel. Suspension of pulp having a concentration preferably exceeding 8 per cent of cellulose containing fiber material is continuously fed to a bleaching vessel and treated with an acid to adjust the pH value below 7 and is subsequently bleached in a bleaching stage to a brightness exceeding 75 per cent ISO. Peroxide bleaching takes place at elevated temperature and that the pressure in a bleaching vessel which exceeds two bar and where the cross section of the area the bleaching vessels exceeds 3 square meters.
U.S. Pat. No. 5,755,926 of Hankins et al. is directed to an integrating pulping process for recycling waste paper. The method and system includes a mild alkaline pulping process with oxygen and hydrogen peroxide followed by rapid decompression of fibers and hot washing.
U.S. Pat. No. 5,772,845 to Farrington Jr. et al. is directed to a method of making tissue, without the use of a Yankee dryer. The typical Yankee functions of building machine direction and cross direction stretch are replaced by a wet end rush transfer and the throughdrying fabric design, respectively. The products are preferably made with chemi-mechanically treated fibers in at least one layer. It is noted in the ""845 patent that certain methods can introduce curl, kinks and microcompressions into the fiber which decrease fiber to fiber bonding, decrease sheet tensile strength, and increase sheet bulk, stretch, porosity, and softness. Examples of mechanical treatments include flash drying, dry fiberizing and wet high consistency curling. A preferred method for modifying the fibers is taught to be through the use of a shaft disperser. See Col. 5.
U.S. Pat. No. 5,834,095 to Dutkiewicz et al. discloses a treatment process for cellulosic fibers. The process includes treating cellulosic fibers using high temperatures that are effective to result in modifications to the fiber. The fibers are typically heat treated with hot air. Also provided is a cross-linking catalyst to facilitate fiber modification. See Col. 4, lines 1-10.
U.S. Pat. No. 5,858,021 to Sun et al. discloses a treatment process for cellulosic fibers. The process first prepares the cellulosic fibers in a high consistency mixture with water and then adds an alkaline metal hydroxide. The high consistency process has been found to produce cellulosic fibers that are uniformly treated. In the ""021 patent a high energy disperser such as a twin screw disperser, is utilized. Typical conditions for using the disperser include an energy level of about 6 horsepower-day per ton of cellulosic fiber and a feed rate of cellulosic fiber of about 2000 pounds per hour. See Col. 10, lines 13-40.
U.S. Pat. No. 5,997,689 to Bokstrom discloses a method of bleaching secondary fibers. A secondary fiber pulp is first slushed and then transferred at a consistency of 20-40 percent to a disperser wherein the pulp is mechanically treated and treated with oxygen. The pulp is thereafter conveyed to a bleaching tower wherein it is treated with alkali and hydrogen peroxide.
United States Statutory Invention Registration No. H1704 of Wallajapet et al. is directed to a modified cellulose fiber having improved curl. This statutory invention registration describes an oxidized or sulfonated cellulose fiber having a curled, stable structure. The oxidized or sulfonated curled fiber is prepared by a process including treating the fibers in a high energy refiner effective to provide the desired curl properties to the fiber which is used in disposable absorbent products. Typically, the high energy disperser employed is a twin screw disperser. See Col. 8, lines 10-35.
International Publication WO 98/27269 of Kimberly Clark Worldwide, Inc. discloses a process for treating cellulosic fibers using steam explosion that is reported to result in modified cellulosic fibers that exhibit desired properties such as wet curl properties. Aqueous pulp having consistencies of from 25 to 75 percent are contacted with steam from 2-6 minutes and then explosively decompressed. Curl indices of from about 0.2 to about 0.3 are attained. See Example 1 and Table 1.
There is provided in a first aspect of the present invention a process for producing high bulk cellulosic fiber exhibiting a durable elevated curl index including the steps of: (a) concurrently heat-treating, bleaching and convolving cellulosic fiber pulp at elevated temperature and pressure at high consistency in a bleaching liquor, preferably under conditions selected so as to preclude substantial fibrillation and attendant paper strength and fiber bonding development and (b) recovering the pulp wherein the length weighted curl index of the treated fiber is at least about 20% higher than the length weighted curl index of the fiber prior to the heat treatment and convolving thereof. Typically, at least about 20% elevation of the length weighted curl index of the treated fiber persists upon treatment for 30 minutes in a disintegrator at 1% consistency at a temperature of 125xc2x0 F. As will further be discussed below, the laboratory disintegrator is suitably operated at 3000 rpm and is of the type described in TAPPI Standard T205 Sp-95.